Introduce multi-line editing for the inputline

Introduce multi-line editing for the inputline

This introduces the new MultiLineEdit that basically enhances the old InputLine widget by being
able to expand to show more than one line. All non-editing related things have been moved out of the
MultiLineEdit to ease reuse in other widgets. For example, InputWidget-specific keypresses are now handled
in InputWidget rather than MultiLineEdit.

Closes #209, closes #213, fixes #386. Thanks to Squider for input (I saw your patch too late, but I did refactor
things anyway...)